September 2, 2024
University of Tsukuba
Japan Advanced Institute of Science and Technology
Keio University
Japan Science and Technology Agency (JST)
A polaron quasiparticle is what we call a single particle that is a composite of an electron and crystal lattice vibrations. By irradiating a diamond crystal containing impurities known as color centers with ultrashort pulse laser light and precisely measuring the change in its reflectance, we discovered that polarons emerge from around the color centers and cooperate with each other.
When nitrogen exists as an impurity in a diamond crystal, a carbon vacancy can form right next to it. This nitrogen-vacancy (NV) pair, which contributes to the coloration of the diamond, becomes a lattice defect known as a color center. NV centers have the property of changing their quantum state in extremely sensitive response to changes in the temperature and magnetic field of their surrounding environment, and it is hoped that this property can be utilized for high spatial resolution and high-sensitivity sensor functions. While it is known that strain in the crystal lattice around the NV center causes the electron energy levels of the NV center to split, the details, including the interaction mechanism between electrons and lattice strain, have remained largely unclarified.
In this research, we introduced NV centers with a controlled density into the near-surface region of a high-purity diamond crystal in the form of an extremely thin sheet (nanosheet). By irradiating this sheet with a pulse laser and examining the state of the diamond's lattice vibrations, we found that the amplitude of the lattice vibrations was enhanced by a factor of about 13, despite the relatively low density of NV centers. Consequently, when we calculated the charge state around the NV centers using a computational method based on quantum mechanics (first-principles calculations), we found that it was in a state where positive and negative charges were polarized.
A polaron quasiparticle, which is what we call a single particle that is a composite of an electron and crystal lattice vibrations, comes in several types. In diamond, it was thought that the type proposed by Fröhlich about 70 years ago could not be formed. However, the results of our analysis indicate that Fröhlich-type polarons emerge from the NV centers and spread throughout the nanosheet. This research outcome paves the way for new strategies in quantum sensing technology based on NV centers that utilize polarons.
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